The bottom-sediment environment around river mouths in inner bays is a complex system consisting of physical, chemical and biological processes affected by the deposition of sediment discharge from rivers, the deposition of particulate organic matter created by primary production in the water column and the decomposition of this matter in the bottom sediments. The bottom-sediment environment around river mouths has not been fully understood due to the strong impact of non-steady or intermittent events such as river floods on the environmental system around the area.
　This study aims to investigate the temporal and spatial variations in the bottom sediment characteristics, and impacts of flood event on sedimentary environment and benthic biota around the river mouth. The data showed that the responses of the sediment properties to the impact of Tama River flood during the attack of the typhoon in early September of 2007 were quite different among the monitoring sites. The drastic changes in water content and organic content were observed, especially at the deeper offshore site during the attack of the typhoon. The main reasons for the changes in the sediment properties are considered to be the hydrodynamic agitation of the bottom and the sediment discharge through the Tama River with their subsequent deposition off the river mouth during the typhoon event. It took about 2 years for the sediment properties to recover to the condition before the event. The recovery of the sediment properties in the offshore region and shows that the long-term evolution of the sediment properties was caused by the constant settling of newly deposited suspended particles from the water column; additionally, the accumulation rate was estimated to be 2.5 cm per year. On the other hand, the seasonal variations in sediment properties were dominant in the shallower river mouth area and the change in the properties affected by the typhoon was quickly recovered in several months. Furthermore, the diversity and biomass of the benthic species were increased after the flood especially in the off shore of the river mouth. The re-colonization of the benthic biota is considered to be related with the favorable change in the sedimentary environment for the species with disappearance of fluff layer on the bottom surface by the flood event.